1. Field of the Invention
This invention relates to an electrical connector arrangement for a circuit board package, and to a method of assembling electrical connectors to a circuit board. More specifically the invention relates to an arrangement and method for increasing the spacing between contact tails extending from edge connectors of a circuit board module at the point of electrical interconnection between the circuit board and the contact tails.
2. Discussion of Related Art
The problem of electrically connecting complex control circuitry, requiring hundreds of digital input/output lines, to buses or multiple wire cables, has led to the development of the line replaceable module (LRM). The LRM is a standardized electrical connector which contains several hundred electrical contacts arranged in multiple rows, each contact having a solder tail connected to a pad on a circuit board, the circuit board being situated within a chassis or rack.
A principal application of the LRM is in avionics, where space, reliability, and ease of replacement are critical. LRMS serve as circuit modules for aircraft flight control circuitry, and include connectors for interconnecting flight control circuitry situated in separate modules arranged to be mounted in a chassis or rack and interconnected via mating connectors affixed to the backplane of the chassis or rack and buses or cables designed to carry high baud rate digital transmissions to and from the control circuits.
Because of the need for replaceability, and because the space available for the modules is limited, both the size of the circuit board and the size of the connectors are limited, with the LRM standard known as SEM-E, for example, requiring a circuit board having a width of 5.250 inches and a total connector area of 5.44 by 0.580 inches. Even in such a small space, existing technology is capable of packing 360 contacts, providing high density connections suitable for use in most prior systems.
As aircraft become more dependent on electronics, and the density of the electronic circuits themselves has increased through the use of large scale integration, the standard LRM configurations are no longer capable of reliably providing the required number of input/output lines. In order to fully utilize the capacities of large scale circuit integration, the number of required input/output lines currently approaches 500, which is not possible in existing LRM configurations without sacrificing electrical performance and reliability or increasing the size of the interface connectors for the circuit module.
In order to pack 360 contacts in a space as small as that permitted by the SEM-E standard, which is already an impressive accomplishment by ordinary connector design standards, most previous design efforts have focussed on increasing the number of contacts in the connectors, and varying the geometry of the contact tails to most efficiently utilize the available space while maintaining as much symmetry as possible. Thanks to these design efforts, it would actually be possible to pack more than 360 contacts into an SEM-E module, if not for a heretofore inescapable geometrical limitation, namely the limitation that no matter how many rows of contacts are provided for each circuit board interface, all of the contacts must ultimately be connected to the circuit board itself, and the circuit board is a planar structure. Too close a spacing of the contact tails on the circuit board creates intractable assembly problems in aligning the contacts and maintaining uniform separation.
At present, therefore, the main reason that the number of contacts is limited is not the size or arrangement of the contacts in the module connectors, but rather difficulties in electrically connecting the contacts to the circuit boards in the module. While the number of contacts in the connector itself can been increased by increasing the number of rows and using different length contact tails to reach the circuit board, all of the contacts must still be connected to the circuit board itself.
Of course, one solution would be to improve termination techniques so that the terminations could be placed closer together, but even if this were possible, assembly costs would be greatly increased and reliability and electrical performance inevitably affected. Conventional mass soldering techniques permit a solder pad spacing on the circuit board of approximately 0.025 inches, but at higher densities, maintaining separation and accurate spacing between the connections becomes increasingly difficult.
While staggering of the solder pads, i.e., arranging the solder pads into multiple rows, increases the potential contact density, there is limited space on the circuit board for multiple rows, and the use of different length contact tails, even in otherwise symmetrical configurations, presents a number of problems from both an electrical and manufacturing standpoint, including problems of impedance variability and crosstalk between the contact circuits. In addition, these configurations lack sufficient dimensional misalignment compensation or compliance in the contacts to compensate for tolerances in mounting the connector assemblies on the backplane and for float which occurs in conductively cooled modules after the modules have been mated to the backplane and the heatsink is translated normal to the plane of the heatsink until its edges are in intimate contact with the cold walls of a rack or chassis in which the module is mounted.
The conventional LRM configuration is schematically illustrated in FIG. 1. Termination of the connector contacts to the circuit board package, which is in the form of two printed circuit boards 8 and 9 separated by a heatsink 10, is by means of relative long and thin surface mount leads 7 extending from connectors 11 and 12. The leads are formed into a compliant state with the mating ends of the contacts (not shown) being staggered to provide increased contact density within the connector. The compliant form of the contacts permits the contacts to absorb dimensional differences between the connector and the board package. Examples of this type of arrangement are illustrated in U.S. Pat. Nos. 4,734,042 (Martens et al.), 4,808,115 (Norton et al.), 5,090,116 (Henschen et al.), and 5,090,911 (Welsh).
In order to increase the contact density, as indicated above, not only the contacts within the connector, but also the solder pads or terminations on the circuit board can be staggered, as illustrated in U.S. Pat. Nos. 4,992,052 (Verhoeven), 5,308,248 (Davidge et al. ), and 5,342,208 (Kobayashi et al.), but such staggering takes up space on the circuit board which could otherwise be used for electrical components and circuitry, and does not eliminate electrical problems resulting from necessary variations in the geometry and lengths of the contact tails.
Whether just the mating ends of the contacts in the connector are staggered, or both the mating ends and the solder pads or terminations, each of these prior arrangements suffers from the common disadvantages of having solder tails of different lengths and shapes, resulting in impedance variability and crosstalk problems, and of severely limited float, which is the ability to compensate for tolerances in the positioning of the connectors relative to the circuit board.
These problems, which increase with the density of output lines and contacts, affect both the ability of the connectors to handle high speed digital data transmissions, and the reliability of the modules, and are unacceptable in critical avionics applications, or in other military, industrial, and commercial electronics packaging applications where LRM type circuit board interfaces might be used. Without a solution to these problems, in order to fully utilize the capabilities of large scale circuit integration, the existing standards would simply have to be abandoned and more space would need to be provided for the circuit board, requiring re-design of the components in which the modules are housed, a lack of backward compatibility, and significant increases in overall cost of the systems in which the modules are used.